1. Field of the Invention
The present invention relates to the composition of a glass with the characteristics of lead crystal, but without the presence of lead, and more particularly to lead-free glass compositions especially suitable for the manufacturing of tableware and/or decorative crystal glassware articles of manufacture, which compositions are generally inexpensive and of improved workability and fusibility over a wide range of temperatures, and which articles are essentially not subject to devitrification at a temperature of about 650-1100.degree. C.
All parts or percentages (%) in the compositions herein are by weight (% by weight) unless otherwise specifically indicated.
2. Description of the Prior Art
Lead crystal is well known as a glass characterized in particular by exceptional brilliance, refractive index, density and sonority.
According to the definition given by EEC standard 69/493--adopted in Italian Law No. 827 of 26/11/73, Annex b, columns e and f, line 2--for a glass to be called "lead crystal" it must among other things, i.e. as well as containing at least 24% of lead, have a density (d) greater than 2.90 g/cm.sup.3 and a refractive index (n.sub.d) greater than 1.545. These characteristics are insured by the lead content, which also endows the glass with good workability, sonority and gloss.
The need to invent a new glass with the same characteristics as crystal (i.e. lead crystal) arises from the fact that lead has already been banished from some production processes because of its potential toxicity and it is possible that this restriction will be extended in the near future.
For this reason, alternatives have been investigated for obtaining glasses without lead but with the physical characteristics of "crystal" stated above. To achieve this, the lead can be replaced by a mixture of elements such as barium, zinc, calcium, strontium, bismuth, titanium, and others which are less toxic and increase the values of density and refractive index. However, along with these properties, many of these elements have a high price and can impair the workability or color of the glass, so that their use can be unsuitable in industrial applications.
Researchers therefore have endeavored to determine the most favorable composition for obtaining "lead-free crystals" with characteristics suitable for both craft work and industrial processing, as for example the glass that is the subject of International Application WO 93/16964 (i.e. corresponding to EP 0 553 586 A1) in the name of Compagnie des Cristalleries de Baccarat, with the composition given in the following Table A.
This composition is characterized by replacement of the lead with a mixture of zinc, strontium and calcium, and the corresponding glass has a satisfactory density and refractive index, but it has proved to be difficult to work, as it has a high melting point and softening point and a devitrification temperature that is unsuitable for continuous melting.
TABLE A Oxides % By Weight SiO.sub.2 53-58 Li.sub.2 O 0-0.3 Na.sub.2 O 4.5-7.5 K.sub.2 O 6-10 SrO 0-12 BaO -- ZnO 16-21 SrO + CaO + ZnO 26.5-31 Al.sub.2 O.sub.3 0-1.5 Sb.sub.2 O.sub.3 0.5-1.5 TiO.sub.2 0-2 CaO 0-9 SnO.sub.2 0-2.5 B.sub.2 O.sub.3 0-1.2 La.sub.2 O.sub.3 0-3
Another lead-free crystal is that envisaged in the European Patent Application published under No. 0 594 422 A1 in the name of Toyo Glass Co. Ltd., with the composition according to the following Table B. According to the invention of this European application, the lead is replaced by a mixture of barium, zinc, titanium and boron, and the glass has satisfactory values of the physical parameters, in particular density and refractive index, at reasonable cost.
TABLE B Oxides % By Weight SiO.sub.2 50.0-60.0 CaO 1.0-5.0 BaO 10.0-15.0 Na.sub.2 O 3.0-10.0 K.sub.2 O 5.0-13.0 Li.sub.2 O 0-1.0 TiO.sub.2 5.0-8.0 ZnO 5.0-10.0 ZrO.sub.2 0-2.0 B.sub.2 O.sub.3 1.0-2.0 Sb.sub.2 O.sub.3 0.3-0.8
However, glasses with the chemical composition given in this Table B can, because of the high titanium content, be difficult to decolorize and to work by hand.
A lead-free glass exhibiting characteristics of crystal is disclosed in U.S. Pat. No. 5,468,693 to Comte (Comte-693). The glass disclosed therein includes BaO and ZnO among the main components. Broad ranges of BaO (&gt;13-21% by weight) and ZnO (5-15% by weight), together with SrO (0-&lt;8% by weight, totalling 27-34% by weight SrO+BaO+ZnO), are disclosed, but substantially narrower preferred ranges of BaO (15-19% by weight) and ZnO (8-13% by weight), together with SrO (0-3% by weight, totalling 27-32% by weight SrO+BaO+ZnO), are shown to meet actually the required standards and quality features, the noted broader ranges being unsuitable to reach the requested density and/or refractive index values (see Table 4 hereinafter).
In addition to the need for avoiding the presence of lead while still keeping the required density (greater than 2.90 g/cm.sup.3) and refractive index (greater than 1.545), additional requisites must be met by the lead-free glass, especially when it is used for the manufacture of tableware or the like. In particular, release of toxic compounds from the final article of manufacture produced with the lead-free composition must be avoided. Release of barium, which is the most dangerous (i.e. toxic) substance among those selected by the present invention, as noted hereinafter, must be reduced as far as possible. Additionally, devitrification must be avoided.
Devitrification is the formation of crystals of chemical compounds inside the liquid mass of fused glass, i.e. it is the removal of transparency from the glass due to crystallization. Depending upon the chemical composition of the glass, devitrification may start at different temperatures. The viscosity of the glass is a direct function of the temperature. It can thus be said that devitrification can occur at different values of viscosity depending upon the chemical composition of the glass.
Devitrification is detrimental and highly undesired. The presence of chemical crystals inside the otherwise amorphous mass of the eventually solidified, i.e. crystal, glass represents a defect in the final article of manufacture. Articles including crystals in the amorphous mass must be rejected, since they do not meet the high quality standards applied to this kind of products.
Glass compositions for the production of tableware articles having a unit weight variable in a wide range (e.g. from 50 gr to 3,000 gr and more) should be such that no devitrification occurs. In particular, for the production of articles of manufacture having high weight, such as vases or the like, the mass of fused glass must be workable without the risk of devitrification, down to rather low temperature values (i.e. viscosity values higher than 100,000 poises).
The viscosity value at which devitrification occurs is therefore an essential parameter of lead-free glass compositions. The higher the viscosity value (measured in poises), the better the glass composition is for the application in the production of glass crystal.
Release of barium from the solidified glass is determined by measuring, by means of flame atomic absorption spectra, the barium content in the extracting solution resulting from the application of the USP 23 standard.
It should be noted that small percentage variations in the compounds of the glass may lead to unforeseeable positive or negative effects on the most important parameters which must be taken into account when selecting a proper composition for the production of lead-free glass crystal, namely: density, index of refraction, devitrification, and release of toxic compounds. Positive effects on one of these parameters might be accompanied by undesired values for the other parameters.
Lead-free glasses have also been developed for other uses.
U.S. Pat. No. 4,106,946 to Ritze (Ritze-946) discloses a steep absorption edge filter glass containing at least 0.5% by weight of CdO, CdTe, ZnS, CdS, S and/or Se as colloidal coloring components for providing optical glass filters. Although the Ritze-946 glass does not contain lead, it constitutes a colored glass which is unsuitable for use in providing (generally colorless) lead-free crystal tableware and/or decorative crystal glassware articles
U.S. Pat. No. 4,472,030 to Tachibana et al. (Tachibana-030) discloses a cesium containing optical glass, e.g. for the production of lenses, having 6-32% by weight of Cs.sub.2 O. The Tachibana-030 optical glass contemplates 0-2.5% by weight of PbO but requires the stated amount of Cs.sub.2 O since, if less than 6% by weight of Cs.sub.2 O is present the desired refractive index distribution of the glass cannot be obtained, whereas if more than 32% by weight of Cs.sub.2 O is present the durability of the glass is reduced. The cesium ion concentration of the lens forming the optical glass gradually decreases radially outwardly from the central axis while a counterpart potassium ion concentration gradually increases in such direction and thus the glass is of non-constant and non-uniform quality and properties throughout its mass or extent.
This known glass of Tachibana-030 is hence characterized by a high content of cesium and has therefore a composition which is different from that of the present invention, i.e. wherein a constant and uniform concentration of the ions therein throughout the extent of the glass is contemplated as noted below, and which known glass would be unsuitable for the production of tableware or the like as contemplated for the glass of the present invention.